array_flip

array_flip — Exchanges all keys with their associated values in an array

Description

arrayarray_flip
( array$trans
)

array_flip() returns an array in flip
order, i.e. keys from trans become values and values
from trans become keys.

Note that the values of trans need to be valid
keys, i.e. they need to be either integer or
string. A warning will be emitted if a value has the wrong
type, and the key/value pair in question will not be
flipped.

If a value has several occurrences, the latest key will be
used as its values, and all others will be lost.

User Contributed Notes 13 notes

I find this function vey useful when you have a big array and you want to know if a given value is in the array. in_array in fact becomes quite slow in such a case, but you can flip the big array and then use isset to obtain the same result in a much faster way.

I needed a way to flip a multidimensional array and came up with this function to accomplish the task. I hope it helps someone else.

<?php
function multi_array_flip($arrayIn, $DesiredKey, $DesiredKey2=false, $OrigKeyName=false) {
$ArrayOut=array();
foreach ($arrayIn as $Key=>$Value)
{
// If there is an original key that need to be preserved as data in the new array then do that if requested ($OrigKeyName=true)
if ($OrigKeyName) $Value[$OrigKeyName]=$Key;
// Require a string value in the data part of the array that is keyed to $DesiredKey
if (!is_string($Value[$DesiredKey])) return false;

// If $DesiredKey2 was specified then assume a multidimensional array is desired and build it
if (is_string($DesiredKey2))
{
// Require a string value in the data part of the array that is keyed to $DesiredKey2
if (!is_string($Value[$DesiredKey2])) return false;

Differences are obvious: returns an array of [i]all[/i] of the longest strings, instead of just picking one arbitrarily. Doesn't do the stripslashing or magic stuff because that's another job for for another function.

When you do array_flip, it takes the last key accurence for each value, but be aware that keys order in flipped array will be in the order, values were first seen in original array. For example, array:

In my application I needed to find five most recently commented entries. I had a sorted comment-id => entry-id array, and what popped in my mind is just do array_flip($array), and I thought I now would have last five entries in the array as most recently commented entry => comment pairs. In fact it wasn't (see above, as it is the order of values used). To achieve what I need I came up with the following (in case someone will need to do something like that):

First, we need a way to flip an array, taking the first encountered key for each of values in array. You can do it with:

From an algorithmic efficiency standpoint, building an entire array of lengths to then sort to only retrieve the longest value is unnecessary work. The following should be O(n) instead of O(n log n). It could also be:

I wanted to know if it would return false and/or even chuck out an error if there were no key-value pairs to flip, despite being non-intuitive if that were the case. But (of course) everything works as expected. Just a head's up for the paranoid.

I know a lot of people want a function to remove a key by value from an array. I saw solutions that iterate(!) though the whole array comparing value by value and then unsetting that value's key. PHP has a built-in function for pretty much everything (heard it will even cook you breakfast), so if you think "wouldn't it be cool if PHP had a function to do that...", odds are it already has. Check out this example. It takes a value, gets all keys for that value if it has duplicates, unsets them all, and returns a reindexed array.